1. Field of the Invention
The invention relates to a device for diagnosing an abnormality of an air-fuel ratio sensor that detects the air-fuel ratio of exhaust gas of an internal combustion engine, and a method for the abnormality diagnosis.
2. Description of the Related Art
In an internal combustion engine equipped with an exhaust gas purification system that uses a catalyst, it is indispensable to control the mixing proportion between air and fuel in a mixture that is burned in the internal combustion engine, that is, the air-fuel ratio, in order to effectively remove harmful components of exhaust gas through the use of the catalyst. In order to perform such a control of the air-fuel ratio, an air-fuel ratio sensor that detects the air-fuel ratio on the basis of the concentration of a specific component of exhaust gas is provided on an exhaust passageway of the internal combustion engine, and a feedback control is carried out so as to bring the detected air-fuel ratio closer to a predetermined target air-fuel ratio.
Incidentally, if the air-fuel ratio sensor has an abnormality, such as degradation, a failure, etc., the air-fuel ratio feedback control cannot be accurately executed, so that the exhaust gas emission deteriorates. Therefore, diagnosis of abnormality of the air-fuel ratio sensor has been conventionally conducted. Particularly, in the case of engines mounted in motor vehicles, the detection of abnormality of the air-fuel ratio sensor in a vehicle-mounted state (on board) is required by laws or regulations or the like in many countries in order to prevent the motor vehicles from running while emitting deteriorated exhaust gas.
Japanese Patent Application Publication No. 2005-30358 (JP-A-2005-30358) discloses an abnormality detection device for an air-fuel ratio sensor that cyclically increases and decreases the air-fuel ratio through an open-loop control and that detects an abnormality of the air-fuel ratio sensor on the basis of the length or area of the locus of the air-fuel ratio sensor output that increases and decreases corresponding to the cyclical increases and decreases in the air-fuel ratio. Besides, Japanese Patent Application Publication No. 2004-360591 (JP-A-2004-360591) discloses that the system from the injection of fuel to the output of the air-fuel ratio sensor downstream of the catalyst is modeled, and a transfer function of this model is represented by a first order response delay element and a waste time element, and the air-fuel ratio feedback control gain is recursively changed on the basis of identified parameters (a proportional constant, a time constant of delay, a waste time) that are recursively identified in this model. The identified waste time is also used for the degradation diagnosis of the catalyst.
However, the technology described in Japanese Patent Application Publication No. 2005-30358 (JP-A-2005-30358), although being able to specifically determine abnormality of the air-fuel ratio sensor itself, is not able to specifically determine details of the abnormality. The technology described in Japanese Patent Application Publication No. 2004-360591 (JP-A-2004-360591) is intended to optimize the control gain of the air-fuel ratio feedback control, and is therefore able to perform the degradation diagnosis of the catalyst in association with the optimization, but is not able to perform diagnosis of abnormality of the air-fuel ratio sensor.
Incidentally, with regard to abnormality of the air-fuel ratio sensor, there is a problem of mis-installation in which the mount position of the sensor is mistaken at the time of mounting or replacing the sensor. If this mis-installation occurs, inability to execute a required air-fuel ratio control results and the exhaust gas emission deteriorates, as in the case where the sensor itself is abnormal.